Shafts of a gas turbine and a supercharger (turbocharger) are driven to rotate at high speed. Further, turbine blades mounted to the shafts are exposed to high temperature. Thus, bearings for supporting those shafts are required to endure severe environments involving high temperature and high speed rotation. As bearings for such use, an oil-lubricated rolling bearing or a hydraulic dynamic pressure bearing may be used. However, use of the hydraulic bearings is restricted under such conditions that lubrication with a liquid such as a lubricating oil is difficult, that an auxiliary device of a lubricating oil circulatory system is difficult to provide separately in view of energy efficiency, and that shearing resistance of the liquid causes problems. Under the circumstance, attention has been focused on an air dynamic pressure bearing as a bearing suited to use under the above-mentioned conditions.
In general, the air dynamic pressure bearing has rigid bearing surfaces on both of a rotary side and a fixed side. However, in the air dynamic pressure bearing of this type, when stability limit is exceeded under a state in which management of bearing gaps that are formed between the bearing surfaces on the rotary side and the fixed side is insufficient, self-excited centrifugal whirling of a shaft, which is called a whirl, is liable to occur. Thus, it is important to manage the gaps in accordance with operating rotational speeds. However, in environments involving drastic temperature changes as in the case of the gas turbine and the supercharger, widths of the bearing gaps fluctuate due to influence of thermal expansion, and hence the gaps are significantly difficult to manage with high accuracy.
There has been known a foil bearing as a bearing that allows the gaps to be easily managed even in the environments involving drastic temperature changes. The foil bearing refers to a bearing having bearing surfaces formed of flexible thin films (foils) having low flexural rigidity. The foils of the foil bearing are flexible, and hence appropriate bearing gaps are formed in accordance with operating conditions such as a rotation speed of a shaft, a load on the shaft, and an ambient temperature. In this way, the foil bearing has a feature of excellent stability, and hence can be used at higher speed in comparison with general air dynamic pressure bearings. Further, in the general dynamic pressure bearings, bearing gaps of approximately several micrometers need to be constantly secured. Thus, in consideration of not only a manufacturing tolerance but also the thermal expansion in the drastic temperature changes, the gaps are difficult to strictly manage. Meanwhile, the foil bearing is advantageous in that bearing gaps only need to be managed to have a size of approximately several tens of micrometers, and hence the foil bearing can be easily manufactured and the bearing gaps can be easily managed.
Further, reactive forces generated in thrust directions by the high speed rotation of the turbine are applied to the shafts of the gas turbine and the supercharger, and hence the shafts need to be supported not only in a radial direction but also in the thrust directions. For example, Patent Literatures 1 to 3 each disclose a foil bearing for supporting a rotary shaft in a radial direction. Further, Patent Literatures 4 to 6 each disclose a leaf type thrust foil bearing as one type of a thrust foil bearing for supporting a rotary member in a thrust direction. The thrust foil bearing includes a plurality of leaves provided at a plurality of positions in a circumferential direction of an end surface of a fixed member. The leaves each include one circumferential end provided as a free end, and another circumferential end fixed to the end surface of the fixed member. Along with rotation of the rotary member, a thrust bearing gap is formed between bearing surfaces of the leaves and an opposing end surface of the rotary member, and a fluid film in the thrust bearing gap supports the rotary member in the thrust direction in a non-contact manner.